The present invention relates generally to manufacture and repair of machine parts, and, more specifically, to surface finishing of such parts.
Machines are assemblies of various parts which are individually manufactured and assembled. Machines typically include metal parts, although synthetic and composite parts may also be used. And, each part requires specialized manufacturing.
For example, metal parts may be fabricated from, metal stock in the form of sheets, plates, bars, and rods. Metal parts may also be formed by casting or forging. Such parts may be machined to shape in various manners.
Machining requires the selective removal of material to configure the part to its final shape and size within suitable manufacturing tolerances, typically expressed in mils, and with a suitable surface finish which is typically smooth or polished without blemish.
Each step in the manufacturing process of a given machine adds time and expense which should be minimized for producing a competitively priced product. It is desirable for each subsequent step in the manufacturing process to avoid damaging previously finished portions of the part which would then require additional corrective finishing steps.
Gas turbine engines are an example of a complex machine having many parts requiring precise manufacturing tolerances and fine surface finishes. A typical engine includes a multistage compressor for pressurizing air which is mixed with fuel in a combustor and ignited for generating hot combustion gases which flow downstream through one or more turbine stages that extract energy therefrom. A high pressure turbine powers the compressor, and a low pressure turbine provides output power, such as powering a fan disposed upstream from the compressor in an aircraft engine application.
The engine thusly includes various stationary components, and various rotating components which are typically formed of high strength, state of the art metal and composite materials. The various parts undergo several steps in their manufacturing and are relatively expensive to produce.
Many of these parts are in the form of annular casings having one or more rows of bosses. A typical boss is a raised cylindrical protrusion extending radially outwardly from the surrounding annular surface of the casing. The casing, including its many bosses, may be fabricated or cast to substantially its final size and surface finish except for final machining of the bosses.
For example, a typical compressor casing has many rows of many bosses used for supporting corresponding variable compressor vanes pivotally mounted therein. Each row of bosses is initially cast with excess material, around the common diameter thereof, which excess material is removed in a vertical turning lathe to the required final outer diameter of the boss row.
Each boss is initially solid as cast, and requires subsequent drilling for forming a through-hole in which the spindle of the corresponding compressor vane is later inserted during assembly.
The turning operation typically forms sharp metal burrs along the trailing edges of the bosses, relative to the direction of turning, with the leading edges typically having a relatively sharp 90xc2x0 corner.
Deburring is required for removing the undesirable burrs, and the remaining sharp are preferably radiused for removing extraneous material therearound.
Since deburring and radiusing are desired around the perimeter edges of each of the several bosses in each of the several axial rows, the geometrical complexity thereof renders impractical automated processing, and therefore deburring and radiusing are typically done by hand. One advantage of hand processing is that the surrounding pre-finished surface of the casing is readily protected from any additional material removal therefrom.
But, a significant disadvantage of hand processing is the corresponding amount of time and labor cost associated therewith. And, hand processing is subject to the skill of the operator and performance of the hand-held grinding tools typically utilized which can introduce undesirable non-uniformity from boss to boss. In the worst case, a boss may be damaged beyond repair, which requires scrapping the entire part, with a corresponding loss of money.
Accordingly, it is desired to provide an improved process for treating a workpiece having bosses protruding from a surrounding surface without affecting surface finish thereof.
A fluid stream is discharged along a workpiece surface toward a boss thereof to form a boundary layer atop the surface. A stream of pliant shot is scrubbed across the boss for selectively abrading target material therefrom while the boundary layer protects the surface from abrasion.